A digital Holter recorder was modified and assembled with a dedicated portable, water- and pressure-proof, hydrodynamic protective case to detect electrocardiograms (ECGs) in marine water on free swimming dolphins (Tursiops truncatus) and human divers using a self-contained underwater breathing apparatus (SCUBA). Employment of a non-invasive appliance leaves subjects awake and not annoyed by the presence of the instrument. Recordings with a good signal-to-noise ratio were obtained, thus revealing electric events throughout the heart cycle of subjects diving at the water surface level and at depth. Results demonstrate that while dolphins show the already known apnoea diving bradycardia, normoxic-air breathing subjects display noteworthy increases in heart rate and atrium ventricular conduction velocity which likely represent functional adjustments to the performing of swimming and diving physical exercise. The instrument appears as a promising diagnostic tool for future studies in the fields of scuba and apnoea diving sports, occupational medicine and veterinary science.